The invention concerns a method for the open-loop and closed-loop control of an internal combustion engine with a common rail system, in which, during normal operation, the rail pressure is controlled by closed-loop control, and, when a load reduction is detected, a change is made from closed-loop control to open-loop control, wherein, during the open-loop control operation, the PWM signal is temporarily set to a PWM value that is higher than in normal operation in order to act on the controlled system.
In a common rail system, a high-pressure pump delivers the fuel from a fuel tank to a rail. The admission cross section to the high-pressure pump is determined by a variable suction throttle. Injectors are connected to the rail. They inject the fuel into the combustion chambers of the internal combustion engine. Since the quality of the combustion is decisively determined by the pressure level in the rail, this pressure is automatically controlled. The closed-loop high-pressure control system comprises a pressure controller, the suction throttle with the high-pressure pump, the rail as the controlled system, and a filter in the feedback path. In this closed-loop high-pressure control system, the controlled variable is the pressure level in the rail. The measured pressure values in the rail are converted by the filter to an actual rail pressure and compared with set rail pressure. The control deviation obtained by this comparison is then converted to a control signal for the suction throttle by the pressure controller. The control signal corresponds, e.g., to a volume flow in the unit of liters/minute. The control signal is typically electrically generated as a PWM (pulse-width-modulated) signal of constant frequency, for example, 50 Hz. The closed-loop high-pressure control system described above is disclosed by DE 103 30 466 B3.
Due to the high dynamic response, a load reduction is an event that is difficult to control from the standpoint of automatic control engineering. For example, after a load reduction, the rail pressure can rise with a pressure gradient of up to 4000 bars/second. If, for example, the internal combustion engine is being operated at a steady rail pressure of 1800 bars, and if the PWM frequency is 50 Hz, corresponding to a period duration of 20 ms, the rail pressure can rise by up to 80 bars before there is a response to the load reduction by the change in the PWM signal. Further complicating the situation is the fact that determination of the pressure signal, computation of the correcting variable, and output of the PWM signal occur at different times. In the most unfavorable case, the resulting lag time can be up to two PWM periods. This lag time is critical, because the maximum rail pressure is limited by a passive pressure control valve, which opens, for example, at 1950 bars.
To improve the reliability of the closed-loop pressure control in a load reduction, DE 10 2005 029 138 B3 proposes that the control operation be changed from closed-loop control to open-loop control. In the open-loop control operation, the PWM signal for activating the suction throttle is temporarily set to an increased PWM value by a step function, which accelerates the closing process of the suction throttle.
To improve the dynamic response for large jumps in set value, DE 40 20 654 A1 proposes that the end of the pulse of the PWM signal or the frequency of the PWM signal track the actual development of the set values and actual values. However, the basic prerequisite for this method is the synchronous start of the PWM signal and the determination of the set/actual values. This method is out of the question for closed-loop pressure control in the common rail system, because asynchronicity of closed-loop pressure control and PWM signal is normally the case. In addition, during a load reduction, frequency tracking in the sense of a frequency increase with a subsequent frequency reduction is not technically feasible due to the high pressure gradient.
To reduce the pressure oscillations in the rail, which are set in motion by the suction valve, DE 103 30 466 B3 proposes a frequency change of the PWM signal. To this end, the angular separation of two injections and the frequency of the PWM signal are used to compute a critical engine speed, at which the frequencies of the PWM signal and the injection are almost equally large, and from this a speed range is defined. If the engine speed passes through this speed range, the PWM signal is changed from a first frequency, for example, 100 Hz, to a second frequency, for example, 120 Hz. As a result of the frequency change, the high-pressure closed-loop control system is stabilized in the range around the critical speeds.
Proceeding from the temporary PWM assignment during a load reduction, as described in DE 10 2005 029 138 B3, the objective of the present invention is to further optimize the closed-loop pressure control during a load reduction.
As described in DE 10 2005 029 138 B3, a first filter is used to compute the first actual rail pressure, from which the control deviation is computed. At the same time, a second actual rail pressure is computed by a second, faster filter. A load reduction is then detected by virtue of the fact that the second actual rail pressure exceeds a first limit. When the load reduction is detected, the PWM signal is then switched from a first frequency, for example, 50 Hz, to a second, mush higher frequency, for example, 500 Hz. If the second actual rail pressure subsequently exceeds a second limit, the operation is changed to open-loop control with the temporary PWM assignment. The optimization thus consists in the fact that the lag time between the detection of the load reduction and the output of the PWM signal is shortened. This has the advantage of a significant reduction of the rail pressure overshoot after the load reduction.
The function is ended then the second actual rail pressure falls back below the first limit reduced by a hysteresis value. When the function ends, the PWM signal is then switched from the second frequency back to the first, lower frequency. Since the higher PWM frequency is set only during a short interval of time, the dissipation and the heat generation of the switching transistors in the electronic engine control unit remains within the specifications given by the semiconductor manufacturer.